High energy density batteries, i.e., those which can produce at least 50 watt hours per pound (for a secondary battery) are desirable in a number of end uses. A number of secondary batteries have been designed to improve energy capacity.
Numerous patents describe the use of aqueous metallic halide solutions as electrolyte with halogens as electroactive materials. One of the earliest patents is U.S. Pat. No. 1,377,722 which describes the use of liquid chlorine under pressure. A good description of such metallic halide secondary batteries can be found in Symons U.S. Pat. No. 3,713,888 which is hereby incorporated by reference. Symons stores the halogen which evolves at the positive electrode during charging by converting the halide into a halogen hydrate. The halogen can also be stored in other forms such as, for example, as the liquid.
Liquifaction of the halogen can be accomplished by employing a much higher than normal pressure at normal temperature, or a much lower than normal temperature at normal pressure, or by employing a moderately higher than normal pressure at moderately lower than normal temperature. For example, U.S. Pat. Nos. 456,843 and 1,377,722 employ much higher than normal pressure and French Pat. No. 133,844 relies on a difference in temperature to achieve liquification.
Each of these alternative methods of halogen liquifaction conventionally require somewhat cumbersome mechanical apparatus for both accomplishing the liquifaction and for isolating the electrochemical elements of the battery from the elevated pressures and/or reduced temperatures used for the liquifaction. In addition to being cumbersome, the conventionally required apparatus can also be very energy wasteful. Accordingly, the use of liquid halogen as a means of practical halogen storage has not been believed to be practical.
It has been discovered that if a halogen gas accumulator is maintained apart from the electrode, and if the metal and halogen electrodes are separated by a cation permselective membrane, and if separate but pressure balance electrolyte flows for the metal and halogen electrodes are utilized, the battery will intrinsically develop a pressure sufficient to liquify the chlorine and there is no need to employ the energy wasteful, cumbersome mechanical apparatus.
Accordingly, it is the object of this invention to provide a system and apparatus for a metal halide secondary battery which will intrinsically develop a pressure sufficient to liquify the halogen generated during charging so that the halogen can be conveniently stored for use during the discharge cycle of the battery.